The present invention relates generally to the field of image forming and in particular to a variable force biasing mechanism and an electrical connection for a removable cartridge unit in an image forming apparatus.
The electrophotographic image forming process is well known in the art. A photoconductive surface, such as a drum, roller, or belt, is uniformly charged to a first voltage level. A latent image is then formed on the photoconductive surface by incident optical energy, such as a laser beam. The latent image is developed by applying toner to the photoconductive surface. The toner is typically applied by a developer roller, the surface of which is charged to a second voltage, with toner electrostatically adhered thereto. The toner is electrostatically transferred from the developer roller to the latent image on the photoconductive surface by the voltage difference between the developer roller surface and the latent image area on the photoconductive surface.
Critical factors in the accurate development of latent images are the force applied along the contact between the developer roller and the photoconductive surface, known in the art as the nip force, and the uniformity of the nip force along the nip or contact area. Among other factors, the optimal nip force is determined by properties of the toner. As the state of the art in toner composition advances, the optimal nip force between the developer roller and photoconductive surface, for a given toner formulation, may change.
The nip force is typically controlled by housing the photoconductive surface, such as a photoconductive drum, and the developer roller in a common replaceable cartridge unit, with the nip force controlled within the cartridge unit by low rate springs. The nip force adjustment is accomplished by altering the low rate springs within the cartridge, so that new cartridges, containing the latest formulation of toner, can be installed in existing machines and function at the latest desired nip force.
In addition to control of the nip force, a recurring challenge in the design of removable cartridge units is the provision of electrical contacts for biasing the photoconductive drum and developer roller surfaces to their required voltages, and in grounding these elements. These contacts should provide reliable electrical connectivity, but exert minimal influence on the carefully controlled nip force. Additionally, electrical contacts may be necessary for a doctor blade and/or toner-adder roller, and possibly characterization electronic circuits.
A recent advance in the design of electrophotographic image forming devices separates the developer roller and the PC drum into distinct removable cartridge units. A removable developer unit stores fresh toner of one color, and includes a developer roller, a toner-adder roller, doctor member and three agitating paddles. A removable cleaner unit contains a photoconductive drum, a charge roller, a toner cleaner unit, and a waste toner auger. Mechanical hardware in the machine housing urges the developer unit against the cleaner unit, generating a nip force between the developer roller and the photoconductive drum. However, since this hardware resides in the machine, it is difficult to adjust the nip force to different values for different developer units.